Diffuse axonal injury is a major component of the motor and cognitive sequela of traumatic brain injury (TBI). Ubiquitin C-terminal hydrolase L1 (UCHL1) is expressed at high concentrations in neurons and may play an important role axonal transport and synaptic function. UCHL1's activity is important in degrading abnormal neuronal protein via the ubiquitin proteasome pathway. Thus, UCHL1 may play an important role in recovery after injury via several mechanisms. In preliminary studies, a TAT-UCHL1 protein was constructed that readily enters the brain when given i.p. Treatment with TAT-UCHL1 reduced axonal injury detected by APP immunocytochemistry and decreased hippocampal cell death after controlled cortical impact (CCI) in mice. A reactive lipid binding site at the 152 cysteine of UCHL1 was identified that is responsible for unfolding and inactivating the enzyme. A knock-in mouse bearing a C152A mutation was constructed and found to have significantly reduced axonal injury and improved motor behavior after CCI. A mouse bearing a C90A mutation with UCHL1 activity that is devoid of hydrolase activity was also constructed. The goals of the current study are: 1) Determine the role of the UCHL1 C152 site in axonal injury, neuronal death, and motor and cognitive behavior after CCI in mice. 2) Determine the role of UCHL1 hydrolase activity in axonal injury, neuronal death, and motor and cognitive behavior after CCI in mice. 3) Test whether systemic treatment with TAT-UCHL1 fusion proteins will reduce axonal injury in the in vitro stretch model and in vivo TBI model, and improve long term motor and cognitive function after CCI in mice. The broad long term objective of these studies is to develop novel approaches that promote axonal preservation and functional recovery after TBI. Completion of these studies will improve scientific knowledge regarding the role of UCHL1 in neuronal repair and functional recovery after TBI and test novel TAT-UCHL1 proteins as a novel strategy to address axonal injury.